Many-body interaction energies and anharmonic OH stretching frequencies have been calculated for water in chain formations, in a ring structure, and in a tetrahedral arrangement. The calculations were of ab initio type, with the electron correlation energy included by Moller-Plesset perturbation correction to second order (MP2) and the basis-set superposition error corrected by the counterpoise procedure. The maximum chain length was seven water molecules, and the ring was five-membered. The molecules were H-bonded head-to-tail. The two- and many-body energies for the chains and ring are all of the same sign (negative), indicating strong cooperativity. The total nonadditivity contribution to the interaction energy is large, 16% for the longest chain and over 18% for the ring. The interaction energy of ah individual chain member with the rest of the chain shows even larger nonadditivity : over 25% for a molecule in the middle of the chain. This quantity should be of relevance for molecular dynamics simulations of liquid water. The OH stretching frequency downshift increases for all members of the chain with increasing chain length and is larger for molecules in the interior of the chain (-357 cm(-1) for the middle molecule in the 7-chain) than for terminal water molecules. The frequency converges only slowly for water molecules in the interior but faster for terminal water molecules. "Frequency cooperativity" was investigated by calculating many-body contributions in a manner analogous to the energy calculations. The chain and ring exhibit strong cooperativity. Infrared absorption intensities and charge transfer were investigated.